There has been an increasing demand to develop tires with a high level of handling performance, good stability and steering response when changing lanes, avoiding obstacles on the road and cornering. Improved road grip without compromising stability is critical for vehicles traveling at high speed. However, higher tire operating temperatures are encountered at high speeds than are experienced during normal driving and the hot rubber in the tire becomes more pliable which reduces the handling stability of the tire, a so-called “borderline” use of said tire.
A widely adopted method to improve stability, particularly road gripping properties, is to increase the hysteresis loss of tread rubber compositions. A large hysteresis loss during the deformation of tread is used for increasing a friction force between the tread and road surface. However, a significant increase of heat buildup will occur during the running of the tires as the hysteresis loss of the tread rubber becomes large, causing wear resistance of the tread rubber to deteriorate rapidly. On the other hand, it is believed that controllability is significantly influenced by hardness (which is closely related to cornering stiffness of a tire) and breaking strength of rubber compositions. In order to enhance controllability, especially steering response, it is necessary to increase the stiffness of the tire compound in general and the tread in particular, which in most cases results in lower hysteresis loss. Therefore, it is very difficult to achieve both of these desired properties by conventional compounding techniques.
In the past, various attempts have been made to improve driving performance by adding a large amount of a softener or a plasticizer to a rubber composition, or increasing the amount of carbon black, employing carbon black with high surface area, or increasing the amount of sulfur. However, none of these methods are successful in achieving desirable driving characteristics without deteriorating abrasion resistance, especially when running at high speed. Tread rubber compounds containing large amounts of a softener show improved driving performance at low temperatures. However, the inclusion of high levels of softeners results in reduced wear resistance and poor processability. The introduction of a large amount of carbon black with high surface area increases the difficulty of filler dispersion in rubber compounds. Furthermore, increasing the amount of sulfur results in reduced hysteretic loss, but leads to poor traction characteristics.
Rubber compositions containing styrene-butadiene copolymer rubber (SBR) with a high content of bound styrene have been used in tread compositions to increase hysteresis loss. However, such rubber compositions can be inadequate in elastic modulus, especially at high temperatures. Accordingly, polystyrene resins, styrene-butadiene block copolymers, and other hydrocarbon resins have added to such rubber compositions for the purpose of increasing elastic modulus (see U.S. Pat. No. 4,487,892; Japanese Patent No. 5,009,338; European Patent No. 0,117,834A1 and European Patent No. 0,470,693A2). However, the aforementioned materials are essentially soluble in SBR base compositions and act as softeners at elevated temperatures. This results in the hardness and dynamic storage modulus decreasing after substantial heat buildup during high-speed running, which in turn lead to poor controllability and reduced wear resistance.
U.S. Pat. No. 6,469,101 discloses a vulcanizable rubber composition comprising (a) cross-linkable unsaturated chain polymeric base, and (b) from 2 to 30 parts by weight of the polymeric base of at least one polymeric organic compound solid at room temperature and having a glass transition temperature of from 80 to 160° C. and an average molecular weight of from 10,000 to 1,000,000, said polymeric organic compound being substantially insoluble in said polymeric base, wherein the rubber composition has, once vulcanized, a first hysteretic behavior at a first working temperature of a tire having a thread made of the composition and a second hysteretic behavior, which differs from the first hysteretic behavior, at a second working temperature of the tire. The benefits of the incorporation of these organic substances are improved low rolling resistance at 40–70° C. and a high hysteresis loss above 70° C. However, it has been found that the significant increase of hysteresis loss due to phase transition of the included organic substances generates high heat buildup as well. In addition, the spike of hysteresis loss can result in a dramatic change in driving performance and reduced handling performance which is commonly known as “give-up”. Without wishing to be bound by any particular theory, it is believed that the nature of the claimed materials determines that they are unable to maintain the integrity of the phase domains formed while the tread is under high thermomechanical stress. The fusion and flowing of the domains occurs during phase transition, resulting in degradation of controllability. Therefore, the method disclosed to improve controllability and stability is not effective to achieve the total objective being sought.
U.S. Pat. No. 6,127,488 discloses that rubber mixtures prepared from at least one styrene/butadiene rubber gel and at least one rubber which contains double bonds and optionally further fillers and rubber auxiliary substances are in particular suitable for the preparation of vulcanizates having unusually high damping at temperatures of from −20 to +20° C. as well as unusually low damping at temperatures of from 40 to 80° C. The rubber mixtures may therefore be used, for example, to manufacture vehicle tires which grip wet road surfaces and have low rolling resistance.
Japanese Patent No. 2002080642 discloses the inclusion of uncrosslinked or crosslinked polyacrylate or polymethyl methacrylate particles with a mean particle diameter of 1–200 micrometers in tread rubber compositions. The benefits touted as being obtained are improved wet traction, better wear resistance and low heat buildup, without degrading dry traction. Japanese Patent No. 2002080642 also disclosed that particles smaller than 1 micrometer are insufficient to reduce hysteresis loss.
On the other hand, nanoparticles are currently receiving significant interest in rubber compositions. They often provide unique physical properties that differ from those of the bulk material. The higher surface to volume ratio of the nanoparticles provides a better opportunity for chemical and physical interactions with the polymer matrix.
The technical problem underlying the present invention is to provide a solution for obtaining a balance between improved controllability, particularly road grip, and good stability for better overall handling performances at high running speed. As previously mentioned, the hysteretic loss of the tread rubber should be large to obtain high gripping force (traction characteristics) between the tread and road surface. High-speed handling performance closely correlates to the stability of the hysteresis loss. Generally, the addition of high Tg material to boost hysteresis loss at high temperature ranges fails to maintain the stability, as the hysteresis loss undergoes sudden change with the phase transition.